Such apparatus is known from GB 2 087 538 and comprises a lock system in the external downcomer forfeeding particles batchwise from the top box back to the bottom box.
EP 0 065 332 and EP 0 451 518 disclose a fluidized bed type heat exchanger in which the fluidized bed is maintained in quasi-stationary, fluidized condition by recirculation of both granular material and medium through an internal downcomer from the top box back to the bottom box.
A further apparatus is known from the international patent application PCT/NL94/00081. In this apparatus, the external downcomer placed outside the reservoir is connected at a top end with the top box via a separator, designed as a cyclone, for separating granular material from the medium. At a bottom end, the downcomer is communicable, through switching on and off, with the bottom box of the reservoir.
A variant of the above apparatus, which is particularly suitable for processing a medium of high viscosity and/or a large volume flow of medium, is described in PCT/NL94/00082. In this apparatus, the bottom box is provided with a divided feed of medium, so that a part of the medium is fed above the fluidized bed, as a result of which the fluidized bed can be stable and sufficiently dense. In this variant, the tubes are provided with run-in pieces, extending into the bottom box, with inflow openings arranged in the tube walls for increasing the inflow of medium.
In the apparatuses known from PCT/NL94/00081 and PCT/NL94/00082, the total amount of flowing medium with fluidized bed particles included therein therefore flows from the vertical tubes of the reservoir via the top box to the separator designed as a cyclone, in which the granular material, hereinafter also called fluidized bed particles, is separated from the medium.
A drawback of this arrangement is that the cyclone, in particular with large throughputs of flowing medium, can assume a considerable dimension and that the walls of the separator and the communication line between the separator and the top box must be protected against wear through the fluidized bed particles present in the flowing medium. The making of arrangements against wear for such a relatively large cyclone separator can be very expensive, in particular when the flowing medium is also corrosive.
Furthermore, since the separator must be located at about the same height as the top box, the large cyclone separator must be supported separately. Since the reservoir with the top box often reaches a considerable height, the support is often relatively complicated and expensive.
Another drawback of a cyclone as separator is the pressure drop required for effecting separation of the fluidized bed particles and flowing medium in the cyclone. When the material of these particles has a low density, e.g. 2400 kg/cm3, and the dimensions of these particles are relatively small, e.g. 1 to 2 mm, this pressure drop can impede the downward transport of the fluidized bed particles in the external downcomer and even cause it to reverse to an undesirable upward transport, as a result of which the fluidized bed particles are no longer fed to the bottom box, but discharged with the flowing medium via the separator.